quick circuit tear down question

[Roger Dingledine]

On Wed, Jan 23, 2008 at 03:47:42PM -0600, Jon McLachlan wrote:
>  Maybe more for developers... but, does anyone know a way to tear down 
> only the last relay on an already constructed anonymous Tor circuit, in 
> such a way that the circuit remains unchanged except for the 
> disappearance of the last hop?  It doesn't seem like this is 
> documented/viable in the ControlPort given the spec @ 
> http://www.torproject.org/svn/trunk/doc/spec/control-spec.txt, but maybe 
> someone knows of a neat or hackish trick?  :)  Or maybe future releases 
> of Tor might...

Check out Section 5.4 of tor-spec.txt, which includes:

   To tear down part of a circuit, the OP may send a RELAY_TRUNCATE cell
   signaling a given OR (Stream ID zero).  That OR sends a DESTROY
   cell to the next node in the circuit, and replies to the OP with a
   RELAY_TRUNCATED cell.

I don't think we've added any interface for this into the control
protocol, because we don't really have a safe use in mind yet. You
can read about the feature in tor-design.pdf under the phrase "leaky
pipe". But somebody needs to do more anonymity and performance analysis
first, to tell us what the tradeoffs are between tearing down part of
a certain and just starting a new one.

Hope that helps,
--Roger